Most of the changes are completely pointless renaming of constructor
arguments so that they do not use the same name as the type members,
but :shrug:

Closes #2015

* Added some missing `noexcept`s on custom destructors.
* Fixed `std::move` being called on a const-reference.
* Initialized `ScopedMessage::m_moved` in class definition, instead
of doing so in constructors explicitly.
* Turned some `enum`s into `enum class`es.
* Initialized `StreamingReporterBase::currentTestCaseInfo` in class
definition.
* Some cleanups in SelfTest code.

Closes #1954

* Successive executions of the same `GENERATE` macro (e.g. because
of a for loop) no longer lead to multiple nested generators.
* The same line can now contain multiple `GENERATE` macros without
issues.

Fixes #1913

Doing some benchmarking with ClangBuildAnalyzer suggests that
compiling Catch2's `SelfTest` spends 10% of the time instantiating
`std::unique_ptr` for some interface types required for registering
and running tests.

The lesser compilation overhead of `Catch::Detail::unique_ptr` should
significantly reduce that time.

The compiled implementation was also changed to use the custom impl,
to avoid having to convert between using `std::unique_ptr` and
`Catch::Detail::unique_ptr`. This will likely also improve the compile
times of the implementation, but that is less important than improving
compilation times of the user's TUs with tests.

As far as I know, only a few users actually use it, but these changes
allow us to avoid including a surprising amount of code in the main
compilation path.